This invention relates to a method and apparatus for the quantitative analysis of a solution containing a plurality of anions by ion exchange chromatography.
Ion exchange chromatography has developed in recent years into a widely used analytical technique for the simultaneous quantitative analysis of mixtures of organic or inorganic anions and cations, or for the separation of chemical compounds that can be converted to an anionic or cationic form. In this technique the solution mixture to be analyzed is introduced into a column containing an ion exchange resin, and the different ions are subsequently eluted therefrom by a suitable eluent, viz. an ionic solution specifically selected for the purpose. The different anions or cations emerge from the ion exchange column at different times which are specific for each anion or cation. The retention times are recorded on a chart using automatic detection. The difference in signal between the eluent background and the anion or cation will produce peaks which are directly related to the concentrations of the anions or cations present in the sample solution.
The application of ion exchange chromatography for the separation and simultaneous quantitative analysis of mixtures of inorganic anions, such as halides, sulfates, phosphates, nitrites, nitrates, etc., as well as of cations, such as the alkali and alkaline earth metal ions, has been severely limited by lack of a suitable detector. However, many of these problems were solved by U.S. Pat. No. 3,920,397 which issued on Nov. 18, 1975. This patent describes a method and apparatus for chromatographic quantitative analysis of ionic species in solution. The solution to be analyzed is first passed into a separator ion exchange resin bed where chromatographic separation of the ions is carried out using a developing reagent or eluent. The eluted ions are then passed through a suppressor or stripper ion exchange resin bed where the developing reagent eluent is acted upon by the ion exchange resin so that it does not reach the detector in highly ionized form along with the separated ion species being analyzed. This step is necessary to prevent the high conductance of the ions in the eluent from "swamping" the much lower conductance of the separated ions to be measured. For example, the developing reagent is converted to a weakly dissociated molecule such as water or is captured on ion exchange sites. The low conductance eluent along with the separated ions is then passed through a detector such as a conductivity cell which can now measure the conductivity of the separated ion species and which provides a signal to associated readout means for preparing a permanent record.
The instruments have found wide acceptance and are used for the detection and accurate determination of anionic and/or cationic admixtures down to the trace level. Many uses are described in Ion Chromatographic Analysis of Environmental Pollutants edited by Sawicki, Mulik and Wittgenstein, Ann Arbor Science Publishers Inc., 1978. However, the instruments do suffer from various disadvantages mainly due to the need for suppression. For example, the large dead volume of the suppressor column requires the use of rather large volumes of solutions. The quality of the signal from the conductometric detection system is dependent upon the degree of exhaustion of the suppressor column during regeneration of the suppressor column. The regeneration cycle introduces high conducting electrolytes into the system which require more eluent pumping time to stabilize the system baseline. The chemical composition of the regenerants are acids or bases necessitating that the column, tubing and valves be of material which is corrosion resistent. The analysis of strongly retained anions requires a higher concentration of eluents which causes the suppressor columns to exhaust more rapidly.